Addition of more flexible items and/or rental system

For example, I rent my property to someone to "agist" his cattle on it. I get x dollars per week, per head of cattle for them to eat my grass but from time to time their number changes, like if a bull comes or something like that.
Items in Akaunting require a purchase and sale cost, which obviously I don't have as it's passive income.

I can obviously set up a recurring invoice but that doesn't really track the number of cattle in any meaningful way for my records and requires manual calculation. Is there a better way?

Another example would be if I had some power tools that rented out (no purchase cost, but an hourly rental fee or similar), or I cut grass and made silage to sell, that doesn't have a "purchase cost" (aside from diesel and tractor costs and labour etc), but does have a sale price. Now I can put the purchase cost as zero, but I feel like there could be a more elegant way of handling many many situations similar to these.

Okay, the number of cattle thing can be set in the invoice so I guess I just have to use purchase cost as the rent due per head?
I can try work with this for now.
But I can't seem to delete this original post...