I read a report a while back about how transfer fees operate in the modern game and I think the notion of installments over months, percentage of sales etc isn't real world (again based on just one article)
Effectively clubs now "re-mortgage" the fee, the article said one Australian bank specialised in it, and take the money at point of sale and repay a % on top.
Granted there is no reason to think every club does this nor that rovers do but when we talk about things like Diaz to Everton or Sammie being upward of 2.5m then the idea of installments seems a slight bit outdated