I still recommend fixing the bug in the design. Using a string data type means that you won't be able to correctly sort and filter by quantity, you won't be able to use it in aggregations, it won't be formatted correctly… You may end up doing more and more workarounds that will take more time than fixing the data model. Also, performance will suffer if you need to run queries against the quantity and you'll have to do it in inefficient ways.
If you want to stick to the validation only, I suggest you check the previous discussion in Validation for numeric values in AX. As I mentioned there, how you need to implement it depends on what exactly you want to do. Remember that users may use various number formats (for example, users in some countries use a decimal points, others use a decimal comma). I guess you'll decide to enforce a particular format, regardless of the format used for actual numeric fields, but it's your decision.
By the way, is the about F&O or AX? You've selected both categories.
