The comments come after Germany's Frank-Walter Steinmeier spoke to President Karzai during a weekend trip to the Afghan capital, Kabul.
"He said to me very clearly: the deal has been negotiated, it would not be amended," Steinmeier told the Bundestag, the lower house of German parliament, adding, "I was pleased that Karzai said very clearly that Afghanistan would in any case sign."
The top German official, however, said the Afghan president had given no indication of the timing.
Berlin has urged President Karzai to sign the agreement without any further delay.
The US and its Western allies have been pressuring Karzai to sign the pact but the Afghan president says the matter should be left to the next government after April elections.
President Karzai has repeatedly said he won't be bullied into signing a security pact with the US.
Under the bilateral security agreement, some 10,000 to 12,000 American troops will stay in Afghanistan till 2024 after the 2014 withdrawal deadline.
Karzai has refused to sign the so-called Bilateral Security Agreement (BSA), saying the Afghan nation cannot approve the agreement without guarantees that the US will halt its deadly airstrikes on residential areas.
They have warned that things will get worse should the US set up permanent military bases in Afghanistan.