What should one do who lost track of the shop owner from whom she bought fabric and paid part of its price, with the remaining balance still owed by her?

If you lost track of the shop owner and do not know him, then donate the remaining amount to some poor people with the intention on behalf of the shop owner. However, if you are able to return it to him, then it is your duty to return his right to him. If you are unable to, then donate it on his behalf with the intention and give it to some poor people with the intention on behalf of its owner, as Allah, the Most High, says: (So fear Allah as much as you are able) [Surat At-Taghabun/16].