Is it halal to take TA/DA allowance for 15 days when the work only required 5 days?

I’m working in a government institution. I’ve to go out to different places for field work frequently, for which i’m entitled to get financial amounts in the form of TA/DA from the government. There is a fixed amount per day. Recently i went for a field work that was officially designated to be carried out in 15 days. But me and my boss had the full knowledge that the work requires not more than 4 or 5 days. And, as expected, i completed it in 5 days. Rest of the time i spent at my home. Now, because of some official formalities, i’ve to claim the TA/DA for the whole 15 days period (there is no provision to take only 2 or 3 days allowance). I would like to know whether it will be halal for me to take the allowance for the whole duration since it has been approved by my boss, or will it be haram since i was really not on government duty on the rest of days.