Will the magnet harm my credit cards?

No.  Bagnetique will not harm your credit cards!  But if you have datastrip cards  you should always keep them in a wallet -- it is very very rare, but demagnetisation can happen if there is direct contact with any magnet over a sustained period of time.  Chip cards are 100% safe from magnetic fields.

The story of magnets and their interactions with datastrips is quite interesting, and there is a lot of misinformation out there -- if you want to get deeper into the details and the science bit, See the long answer in this blog post!