Is taking an additional free amount from a commercial offer when purchasing supplies for a domestic worker, along with adding a small amount, considered a sin that requires the money to be returned to the worker who has traveled and cannot be contacted?

If what the questioner did was with the worker's knowledge and consent, then there is no blame. However, if it was without her knowledge and consent, then the additional money is rightfully the worker's, as it is consequential to the contract. If the questioner only paid a small portion of the price, then she is not entitled to any of the additional amount except in proportion to what she paid. In this case, the questioner must return the money to the worker if she can be reached; otherwise, she should donate it on her behalf. Al-Ghazali mentioned in "Minhaj al-Abidin" that repentance from financial sins requires returning the money to its owner. If that is not possible due to absence or death, then it is obligatory to donate it on their behalf.