To put this one to bed (hopefully):
We sold Brooks for £11.5m up front, split into three instalments. Bournemouth paid £4m in July 2018 with £4m due in Jan 2019 and the final £3.5m due in July this year.
We, as most championship sides do, had cashflow constraints last season so we borrowed the £7.5m in July last year, guaranteed against the £7.5m of future income from Bournemouth. In effect, it means that Bournemouth's second and third installments went straight to the bank.
We won't be writing off any outstanding money from the Brooks deal if we sign Mousset. There's no point.