Hey man, I understand the confusion and would like to correct some notions. Particularly with some comments here conflating socialism with other ideas.

Socialism is simply workers controlling industry. This may mean many things including each business being controlled by it's individual workers, all industry being collectively owned by society as a whole, or other similar ideas. Personally I believe in the former. As long as the people working in a business control said business this is socialism

What if that industry is the rental of tractors? Well, then all workers involved in this business must control the business. If you are the sole worker, the only one operating this business, then you have sole control.

If you were to, let's say, buy 10 tractors and bring on a receptionist to manage calls and schedule tractor usage well then that receptionist would also own, and control, this business as well. Same goes as it grows. Just as a person selling wheat to a grain mill doesn't need to hold any ownership over the grain mill nor the mill over the farm you selling your tractors usage doesn't need those using it to own your tracker.

Socialism does not necessitate the collective ownership of property nor does it mean industrial rental isn't an option. It only necessitates that the workers control industry. This may mean that each individual business operates as it's own entity, controlled by it's individual workers.

Workers seizing the means of production can mean does not mean, necessarily, society owning it collectively nor does It mean all who use it may have ownership.

I feel that socialism and communism often get conflated. Private ownership of property and the means of production is allowed under socialism, just not ownership over industry. You can personally own and rent out a tractor but you cannot personally own a tractor rental company. Under communism, all is owned collectively

Now, the morals of renting are another thing altogether and entirely detached from socialism. Personally, I think it mostly immoral. Under your circumstance I see no issue selling excess time with a tool you use for the majority of the time. The issue comes when a single renter start to pay entirely for maintenance with excess profits. In my opinion, this should grant them partial ownership. Once more though this is detached from socialism entirely

If your question refers to it: Marxism is an analysis of the social structure of market economy. It doesn't work as a guidebook for individual behaviour inside of market economy, but for collective emancipation from a social order that results in exploitation.

Doing so would mean decide collectively/democratically over the collaborative use of the means of production.

"I'm buying some means of production with my money" and "their fields" is a market economy situation.

You don't go from caputalism to socialism by individually changing your personal economic behaviour, but by changing social order.

Marxism is a materialist worldview, not a moral one. I could give my personal opinion, but Marxism can never answer a question that begins with "Is it moral/ethical/permissible to...?"

A tractor is a means of production. Owning a tractor would make you a member of the Petite Bourgeoisie - a person who owns their own means of production, but does not own enough to get by without also working, typically self-employed. Leasing the tractor to others doesn't change this, but it is an example of Rentierism - something that will not exist under Communism. If you owned a thousand tractors, and could live comfortably off of the rent you charged others to use them, that would make you a Capitalist, a member of the Bourgeoisie.

Different people will have different thresholds over whether they think some Rentierism is acceptable. I believe it is fine as long as you remain small-scale and have affordable prices. Others may disagree, believing that either all Rentierism is acceptable until Socialism is achieved or that Rentierism is never acceptable. Marxism cannot give an answer to a moral question, and so the answer is personal.

There are alternatives to owning a tractor yourself. Finding a group of people who would use the tractor is feasible. Setup a democratic system to control the use of the tractor and a system of dues to ensure it is well maintained. Basically treat the tractor as a commons that people can exploit in an ethical manner.

This kind of system can be expanded to all means of production in theory. All capital treated as commons for workers to use.

This has the added benefit of being feasible within the current system, makes it easier for workers to survive, and acting as material evidence for alternative economic systems.

If I saved up my money and bought a tractor

What if I instead offered to plow their fields for them instead

You assume it is necessary to use money to buy a tractor for yourself, you assume "their fields" are owned by "them." What if the state provided you with a tractor and the land? Or even if money were involved, what if the state provided you with the money to buy the tractor and the land?

There would be laws to allow you to hire other people to use the tractor and farm the land, but by law, the surplus of their labor (whatever they planted, farm, sell at market) would belong to them, likewise the surplus of your own labor would belong to you. You could use the surplus (money, goods, what have you) to trade with anyone else.

To me, it sounds like you are describing a situation where because you have some money (to buy a tractor) and other people don't, you gain access to some of the human labor of your neighbors.

Socialism is not a guide on moral behavior under Capitalism, but an argument that organizing along Socialist lines is better for Humanity than Capitalist lines.