I won't debate whether it's racist or not, but it's certainly exploitation. Are there any local trade unionists or workers rights advocates who speak the right language, who can contact these workers - away from the work premises perhaps - to explain to them what their rights are and encourage them to enforce them? I've had a few clients for whom I want to do legal aid work who can't prove their income, and they tell me "Asian businesses don't give wage slips". They're surprised when I tell them they have a legal right to one, and to know that their employer is paying their national insurance.
I get very irked by Asian business owners who come to Britain because they can have a better life than they would in their own relatively impoverished countries, but assume they can flout the laws that every reputable British-born business owner is forced to follow. They can't escape health and safety laws easily because the State enforces these without favour, but workers' rights are not given any priority, either by the State or Trade Union organisers. This is a bigger problem than it looks because a lot of rights are interdependent. If you can't prove what your income is, or that you have paid national insurance, you can be unable to claim social security benefits, end up with a reduced pension when you retire, be unable to get free legal advice under the legal aid scheme, and have trouble getting a bank account, credit, or acommodation (private landlords often want to know how much you're earning so they know if you can afford the rent.)
If this business is employing actual bonded labour, it's a bigger issue. When the Royal Mail contracted out this particular post office, what clauses did it put in the contract? Anything about the business being run legally and in a way that would not bring Royal Mail into disrepute? It might be possible to find out through a Freedom of Information Act request.