Capture the handoff before splitting the total
Ask what the cash was intended to cover. Money given toward today's purchase differs from repayment for a previous one. A useful note says “Dana gave Rui $20 at the shop for the $45 purchase.” It records payer, recipient, date, amount and purpose without a second receipt.
Check the two contributions against the receipt
After the handoff Dana has paid $20 and Rui $25. Together those contributions equal $45. The intended final cost is $22.50 each. Moving $2.50 from Dana to Rui produces that result. Rui does not need to pay another $20 into a shared account to make the cash appear on a statement.
Resolve a missing detail separately
If the two intended to split equally, Dana still owes $2.50. Avoid entering the $20 as another purchase on top of the $45. If nobody remembers the exact cash amount, leave it marked as an estimate until the two people agree.
If Rui used a $50 note and received $5 in change, the purchase is still $45. Record who kept the change if it affected the contributions. When there is disagreement about the cash itself, settle the confirmed amount first and leave the disputed amount open rather than guessing who had which notes.