Based on another question you posted, I assume that your jurisdiction (still) is Florida.
Is it legal for a restaurant to limit the amount of a certain type of
No, it is illegal. Here the subtlety is that --per your description-- server's bank constitutes debt (rather than payment for goods or services), and 31 USC § 5103 reads:
"United States coins and currency [...] are legal tender for all
debts, public charges, taxes, and dues"
Since non-compliance with the restaurant's new policy can lead to termination, I think your best option is to proceed in accordance with Florida statute 448.102(1):
- In writing, bring to the attention of a supervisor or the employer
the policy or practice, affording the employer "a reasonable
opportunity to correct the activity, policy, or practice"; and if
the restaurant dismissed the reasonable opportunity,
- disclose to the "appropriate governmental agency, under oath, in
writing, an activity, policy, or practice of the employer that is in
violation of a law, rule, or regulation", which in this case is 31
USC § 5103.
If you have to resort to step (2), be sure to include evidence --in writing-- about the substance of the new policy, showing how the restaurant treats server's bank as "debt".
Having taken these two steps, if the restaurant terminates you, then this might be grounds for a lawsuit. See Florida statute 448.103.
In other jurisdictions, the approach might have to be different. For instance, in Michigan, the server would need to file a complaint with the state's labor agency on the basis of MCL 408.478(* ) (prohibiting the employer to demand "[a] consideration, as a condition of employment or continuation of employment"). Here, the "consideration" is that the employer restaurant is demanding its servers to perform --without extra pay-- tasks that have nothing to do with a server's job: Acting as a bank cashier, or going to the bank towards securing the ability to comply with the restaurant's new policy.
(*) I couldn't find an equivalent statute in the Florida legislature.