I found this bit in an older CNN piece from Jan 19. It's probably the closest to a official and public German demand, back then, albeit was said conditionally...
“If America will decide that they will bring battle tanks to Ukraine, that will make it easier for Germany,” German Vice Chancellor Robert Habeck told Bloomberg from Davos on Tuesday.
One unnamed US official seems to have confirmed that at least some on the US side saw that as a hard line back then:
“They have us over a barrel,” a senior Biden administration official told CNN Thursday, adding that the Germans are demanding tanks for tanks, and not budging on considering any other offers the US has made to spur Berlin to send the Leopards.
But it seems that lot has changed in the week since then. Press reports of late yesterday and this morning suggest that both US and Germany will be sending main battle tanks to Ukraine, albeit the official announcements will supposedly come later today, and might not include a specific date for the deliveries.