This type if construction is one of the most confusing in English. We native speakers do it instinctively, but I'm struggling to explain how/why so you can understand.
The best way to think of this: it is an event that will have passed at some point in the future. (And, at that time will be referred to in the past tense.)
If you wish to used paid (the past and past participle of pay) I would suggest the present perfect tense here: has paid.
The present perfect is used to convey an unspecified time, which this clearly is. Combined with once, it becomes a conditional (which will have passed at an unspecified time in the future).
Otherwise, I would use the present tense once he pays. By combining once and the present tense we achieve a conditional (which will be in the past at a future point).