Hello @iainhallam
While I can’t advise on the accounting side of this, you could rebill the expenses to the sales invoice an contra these. This would reduce the expenditure without affecting your income.
Although I’ve not tested this, you could possibly enter a negative markup to calculate the 50% split.
There’s more on this feature here: Rebilling Expenses
Alternatively, if you’ve already invoiced them for this, you could use a journal to move the balance.