Trump donated to Cruz in 2014 long AFTER he defeated Dewhurst. At some point, Trump probably realized that donations to ONLY progressives and Marxists would look bad for his own presidential run. So, his Donny-come-lately muddying the stream has been sufficient to fool many voters. Not me. Not a chance.
It would exhibit a modicum of class if, before going full backstabber mode, Cruz had at least the decency to return the money.