Provided a contract or agreement doesn't stipulate otherwise.
A way to think about this is that while the constitution recognizes freedom of speech, signing an NDA results in the exchanges of one's right to disclose certain information in exchange for the privileges and benefits of employment.
What about bankruptcy courts? Repossession? Liquidation auctions? The seizure of assets as compensation for debts incurred is not in and of itself unreasonable or particularly atypical, even when the option of cash exists. Although, depending on legislation and jurisdiction, there may be prescribed limits (e.g. homestead exemptions and limited liability)
With all that said, none of what you've stated proves that the law requires all debts to be accepted in cash in every case regardless of circumstances. What you've offered so far is an asserted reading of the text without any arguments backing up the claim.
