Would have been quicker and easier to answer if you'd mentioned which receiver you have extra.
Although Garmin only lists GTM36 and GTM60 as compatible traffic receivers under accessories for 24xx series they're also compatible with Garmin's FM TMC receivers GTM25, GTM 26 or GTM35 provided the receiver has the appropriate local subscription. It can only also use the HD Radio/FM GTM60 receiver to receive bundled HD traffic in NA, but the GTM60 is compatible with FM traffic worldwide when free service is available or a local subscription is added. In EU it can use a DAB HD receiver GTM70/80 which will also receive free FM/bundled FM subs and worldwide will receive free FM and paid FM with an added local sub.
Model numbers below GTM25 are not compatible with 24xx series. GTM20/21 have the flat 18 pin connecter instead of a mini USB. The even earlier GTM10/11/12 although fitted with a mini USB plug are powered differently [i.e. from the unit itself via the USB connection, they don't have an integrated charger] and they also store subs on the device itself not on the receiver.