I have a somewhat similar problem – The constraints I used work well in the web version but doesn’t work in the Kobo collect app. Please see below the codes I used in the web tool:
The idea is that the production quantity can be any number greater than or equal to zero or -99, the code for missing (don’t know/not sure). The quantity sold should not be greater than the production quantity or -99, the code for missing value. The quantity given as gift should not be greater than the quantity produced less the quantity sold, or -99. The constraint formula becomes complicated given the possibility of entering -99.
Nevertheless, the code works in the web tool. However, the Kobo collect app in android phone does not allow the possibility of entering zero for quantity sold and quantity given as gift.
Grateful for your help.