Is fasting sufficient as an expiation for a binding oath for someone financially incapable, or is it obligatory to sell a portion of the shop's capital to pay it, and is it permissible to give wheat flour as expiation?

The expiation must be paid from the capital if its profit is not disrupted; otherwise, you may switch to fasting. This applies if you need the profit for your essential needs. As for giving flour in expiation for an oath, it is permissible.