Is it forbidden to take the monetary subsistence allowance while sometimes eating meals provided by the employer who pays the allowance, and what are the solutions?

Since you accepted the amount in lieu of sustenance, it is not permissible for you to partake of the meals except with the approval of the officials. Otherwise, it would be considered consuming others' property unlawfully. You must now repent to Allah and return the value of the meals you consumed to the competent authority.