But reneging on his vow not to serve as a minister in a Merkel-led cabinet prompted a wave of criticism from many party members still smarting over his decision to renew the loveless "grand coalition" despite initially pledging to take the SPD into opposition.
Meetings in Berlin take place a few hours after Merkel's conservatives agreed with Martin Schultz's Social Democratic Party to form a coalition government.
But a copy of the coalition agreement obtained by Agence France-Presse indicated that he was able to extract a high price from the conservatives in the final round of talks including control of the powerful finance and foreign ministries, as well as the labor, family affairs, justice and environment portfolios. On Thursday night, outgoing Foreign minister Sigmar Gabriel's lashed out at the SPD leadership for having a lack of respect for party members.
The SPD and CDU both had their worst results for decades in the election, which catapulted the far-right Alternative for Germany (AfD) into parliament for the first time, with 94 seats.
The pitch is created to appeal to his party's 464,000 members before they vote on the deal, which is the final hurdle the SPD needs to pass before the government can be formed.
There was no immediate word on who might become foreign minister instead of Schulz, who also didn't detail any plans for his own future.
September's general election was a disaster for the party, however.
Schulz will also step down as party leader, and his likely successor, Andrea Nahles, said the party would now focus on policy content ahead of the ballot.
In a statement on February 9, Schulz said he thought the discussion about him and his role could hurt support for the new coalition once the SPD members vote.
But failure to secure votes for the new coalition would significantly undercut her political authority in the party and plunge the SDP deeper into crisis.
Those opposed to any deal with Mrs Merkel include a new group within the SPD calling itself NoGroKo (no grand coalition).
Germany's Social Democrats punched above their weight and won. In addition to the controversy over Schulz, Merkel herself has been under fire for giving key ministries to the SPD to extend her 12-year reign over Europe's biggest economy.
After a failed attempt to forge an untried alliance with two smaller parties, Merkel opted to woo back the reluctant SPD-her junior partner for two of her three terms since 2005.