Is it permissible for a trainer to ask for an additional sum to his wage for lunch and dinner, or to ask for a comprehensive sum for accommodation and meals and then spend less than that?

It is permissible for you to agree with the institute on the payment of an allowance for lunch and dinner or all expenses, and the determination of the amount depends on the nature of the agreement. If the amount is designated for your ownership as compensation for sustenance and accommodation, then you may dispose of it as you wish. However, if the employer does not disburse the amount to you except for actual consumption, then you are not permitted to take more than what you will spend, in fulfillment of the condition and contract.