The system automatically assigns the modules to each A level to give you the best results. The order of priority is: Maths grade; Further Maths grade; Maths UMS; Further Maths UMS. You do not have to do anything - it is automatically worked out for you.
The idea of 'cashing in' applies more to the first year of sixth form: schools/colleges will choose not to cash in your A level Maths, because then the system knows that the applied modules from it can be moved to Further Maths if this will give you better grades.
Note that if, for instance, your modules could be arranged so that you got BB or AC, the system would choose AC as it prioritises Maths. I believe you are able to contact the exam board and ask them to change it to BB if this is a requirement for your university offer, but don't take my word for it - your maths teachers / someone at your school should be able to help you out if you are in this situation tomorrow.