A backcharge runs the change order in reverse. The drywall sub leaves a site the painter has to clean; the GC's crew spends four hours on it and deducts the cost from the drywaller's final payment. Legitimate when the contract allows it and the cost is documented; poisonous to the relationship when it's a surprise on the last check. Most subcontract agreements require written notice and a chance to fix the problem before a backcharge is applied.
A worked number
Tile sub's contract $4,200. Sub cracks a newly installed vanity top; replacement $640 plus two hours of the GC's carpenter ($90). Backcharge $730, documented with photos and the replacement receipt. The sub is paid $3,470. On the GC's job ledger, the $730 cost is real and the $730 recovery offsets it; the tile line reads $4,200 as budgeted.
The mistake it hides
Applying it silently. A backcharge with no notice and no receipt is a payment dispute, and in some states a wrongful deduction exposes the GC to prompt-payment penalties. The other mistake is absorbing the cost instead: a GC who never backcharges pays for every sub's mistakes out of margin.
In Obra
Obra has no credit or negative transaction, and a backcharge must never be recorded as income: that would raise cash collected without touching cost. Record the repair as a cost on the project with its receipt, then record the sub's payment at the reduced amount you actually paid, with the notice attached and the deduction named in the concept. The job's cost stays true, and the sub's vendor history shows what happened.