The payback method is always briefed when the fri is given. So know that you will have to pay it all back shouldn't be a surprise if you renege/leave etc. Worse is the Unbriefed trashing of your tax code and subsequent self assessment needed that accumulate fines if not done- move often, don't update your address with the taxman (familiar issues for mil).
The army are right to take it back, the precedent otherwise is if we think you are special enough you can ignore the rules. Where the help should be the amount and timing of the pay back. £25 a month or some such.
The army is cavalier with its pay. It's approach to cea is particularly dodgy. So they should very careful