The partner is distributing vouchers corresponding to 100 € of food: should they be considered as vouchers or cash? What does prevail: the amount or the restriction for its use?

They have to be considered as vouchers.

A voucher has per its intrinsic nature a restriction on what can be purchased / exchanged for.
Therefore, if a voucher has a whatsoever restriction is and remains a voucher.

It has to be considered cash only if it allows to retrieve money from an ATM or can be used to purchase whatever and whenever (unlikely).

ATTENTION: some e-cards have wallets: some restricted (i.e. for food only) and some wallets whose amount can be cashed in an ATM. In this case, the partner should differentiate the two types of modalities.