We are able to do that on an 8824. The sale price is the amount the client received and then there is a separate line to determine the Boot that specifies Cash Received less expenses. Per the IRS instructions:
Include on line 15 the sum of:
Any cash paid to you by the other party;
The FMV of other (not like-kind) property you received, if any; and
Net liabilities assumed by the other party—the excess, if any, of liabilities (including mortgages) assumed by the other party over the total of (a) any liabilities you assumed, (b) cash you paid to the other party, and (c) the FMV of the other (not like-kind) property you gave up.
See the example in the instructions for line 18.
Reduce the sum of the above amounts (but not below zero) by any exchange expenses you incurred.
I would reduce net cash received by the amount paid to Person B (the client issued a separate 1099-S for that)?