現金 and お金: cash and money
Distinguish cash on hand from money more broadly.
Money in a broad sense
お金 (おかね) broadly means money. 「旅行にお金がかかります」 means that travel costs money, not that it must be paid for in banknotes. Distinguish the expense from the payment method.
「お金を使いすぎました」 means spending too much money. Further context is needed to know whether it was spent on goods or services.
Cash as a payment method
現金 (げんきん) means cash, such as notes and coins. 「現金で払いました」 says that payment was made in cash, with で marking the method. The sentence does not give the amount.
「現金を持ってきてください」 asks someone to bring cash, not merely show an account balance. An amount can be specified separately.
The scope of not having it
「お金はありますが、今は現金を持っていません」 means that the speaker has money but is not carrying cash now. Money may be held in an account, so this is not contradictory. Do not expand a lack of cash into having no assets.
「現金が足りません」 means there is not enough cash, not necessarily none at all. The comparison is with the amount needed.
What does a payment notice saying 「現金のみ」 restrict?
Show the answer and why
It restricts the payment method to cash, not the price or purchase quantity.