it is very difficult to have a residence permit for work as a rejected asylum seeker because such an option is generally not permitted by german alien law, only in some special hardship cases. Mostly the Ausländerbehörde tells you to go abroad and ask for a working visum in a German embassy, and sometimes it is really a better option and people who already have a job manage to come back quickly. You can ask the Ausländerbehörde to give you their written agreement to your entering Germany with a working visum before you leave Germany. With this document the procedure in the embassy is much shorter, because they must not ask the Ausländerbehörde for agreement anymore, they must only check if you fulfil all conditions (having all necessary documents for a visum). It is often not easy to get such an agreement but some Ausländerbehörden give them.
Without leaving Germany, the only safe way with a Duldung in the moment is a so called "Beschäftigungsduldung". To apply for it you must have a "normal" (which means risky) Duldung for at least 12 months and work for 18 months min. 35 hours a week fully earning your living - and naturally having identity documents and not having any criminal penalties over a very small limit.
As you are in Germany for less than 18 months inclusive your asylum procedure time, you can' have this good Duldung now. You can only try to continue working with a normal, risky Duldung and hope not to be deported and not to lose your working permission before you fulfil the conditions to apply for Beschäftigungsduldung. It is a risky way.
Please ask for advice in a good councelling center or find a good lawyer for alien law. The advice online can't be sufficient as each case is individual.