"You could say that this is actually something that should be taken for granted," Sigmar Gabriel said after talks with his French counterpart in Paris.
"And how bad must the relationship be if we are already celebrating things that should be taken for granted as breakthroughs in the negotiations?" Gabriel asked.
Chancellor Angela Merkel said the offer made by May at the EU summit in Brussels on Thursday was "a good start" but more needed to be done.
Gabriel took a more negative view, saying: "I am not celebrating yet... I personally can imagine no other result than that European nationals who currently live in Britain as European citizens be allowed to stay, that is the minimum."
Over dinner with her 27 EU counterparts, May promised EU citizens living in Britain that they could stay after Brexit, with permanent rights to health care, education, welfare and pensions equivalent to British nationals.
Gabriel said he wanted to see "more clarity on the substance".
"What health insurance, what retirement rights, what legal status will citizens from Europe have in Britain and the same for British citizens in Europe?" he asked.
"The mere fact that you won't be thrown out is not a convincing breakthrough, as far as I'm concerned."