The pair will go head-to-head in the DFB-Pokal final on Saturday as Lewandowski looks to end his four-year spell at Dortmund on a high by beating his future employers.
Lewandowski announced in January that he had accepted a contract with fierce rivals Bayern.
Martinez is looking forward to teaming up with the Poland international next season at the Allianz Arena, but insists he will not go easy on him this weekend.
"It's great that he's back next year with us," Martinez told Sport-Bild. "But still we are not colleagues - in Berlin he gets from me the full force felt.
"We can then be friends from the summer."
Lewandowski has scored 116 goals for Dortmund since arriving from Lech Poznan, including five against Bayern in seven appearances.
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