Settle a debate

Maybe I'm misunderstanding this, but I think you would just spend the difference between whatever your balance was, and the 630.

If he spends 120, and you immediately send him the 120, it essentially means that YOU spent the 120.
You owed 630. You gave him 120. You now owe 630-120=510.
How much have you spent so far, not including the recent transfer?
This is hard to understand because your method of making accounts even also spends money, so it’s hard to reason about those things individually.

He spent 630 on both of you, so you are in a state where you owe him 315. The other 315 he spent on himself and 315+315=630

One (complicated) way to resolve that is for you to buy 630 of stuff that is for both of you, which would result in you buying 315 of goods for him and thereby pay down the debt to make you even.

Another simpler way is for you to pay him 315 and then you are even.

Since You’ve paid 120 we can modify those two ways based on the payment.

Simple way: Pay him another 195 (that’s 315-120), so you are even again.

Complicated way: Buy 390 of stuff for both of you, half of that stuff (value 195) will be for him, and then you are even.