Making change and being able to judge appropriate uses of money become more difficult as someone enters moderate dementia. Your loved one may also make purchases that are quickly forgotten, then repeated. (That's how you wind up with 14 bottles of white-wine vinegar in the cupboard.)
So it's wise not to want someone with progressing dementia to carry around large amounts of cash or credit cards. At the same time, adults who have always carried a purse or wallet can be upset to be deprived of this habit, making them more agitated.
Better to stock the wallet with harmless items that reassure and make the person with dementia feel like he or she has the real deal. Include a few dollars, as well as copies of innocuous ID cards such as library cards. You can even include expired credit cards (ideally from closed accounts) that look familiar and important but won't be damaging if lost.