Using a Retainer to Part Pay for an Invoice That Isn't in the Same Name

Using a Retainer to Part Pay for an Invoice That Isn't in the Same Name

Hoping someone can help me with the following situation:
  1. Person A has paid me a retainer of 230 to work on his boat.
  2. He has sold the boat to person B but agreed to pay 50 from the retainer towards the final invoice.
  3. Person A wants the remainder refunding to himself.
How do I record this in Books so that:
  1. Person A is physically refunded 180 and hasn't got any unused credits on his account.
  2. Person B's invoice is shown to be reduced by 50
Thanks in advance,
Tom.