ومن اشترى ثوبا على أن يقطعه البائع ويخيطه قميصا أو قباء أو نعلا على أن يحذوها أو يشركها فالبيع فاسد
Translation: Quduri states the sale is defective if the seller is required to cut and sew the sold cloth into a garment or to size or attach straps to sold footwear.
This adds a work or service obligation to the transfer of goods. Under the classical analogical rule, that extra stipulation can invalidate the sale.
Fath al-Qadir, however, reports an istihsan/customary-practice exception for certain commonly ordered footwear work.
Study both the abbreviated ruling and the later commentary qualification. Modern manufacture-to-order contracts require their own legal classification.