All clubs owe transfer fees - if the fee is scheduled to be paid over say 5 years, then it is not a problem.
When a club fails to pay another club as per the agreed schedule then a transfer embargo is usually put in place.
Football has quite strong punishment for clubs that fail to pay other clubs as else the pyramid would soon collapse. For example any club in administration must pay all football debts at 100%, without a discount.
If Munoz was to leave (and he is 29) then IMHO Gusto at Chelsea would be an obvious replacement. Much younger and very forward thinking. Unfortunately i suspect Chelsea will want top dollar despite him not being a regular (they bought him for c£30m).