Washington rotated his slave out of Philly because of such laws[Pennsylvania’s Gradual Abolition Act of 1780.] in respect to the time that he could keep them in Pennsylvania[6 months or longer in the state they would be automatically free].
That he respected the views of Pennsylvania, and that he did not wish to test the legality of keeping them their longer than six months, does not mean that he did not have the better legal argument should the issue ever need to be litigated in court.
That he had them there at all indicated that he believed he could continue slavery in Pennsylvania, even if it required him to jump through some legal technicality hoops.