Record physical pieces, not a stash total
Two separate remnants cannot be treated as one magically joined area. Give each piece a width, length, quantity, and fabric identity.
Preserve useful leftovers
A practical allocation should account for every placed piece and describe remaining rectangular regions without claiming they form one contiguous remnant.
Use the right scope
Check one rectangle with Pieces from Fabric. Reconcile several stock pieces and new fabric with the project planner; no permanent stash database is required.