Hello new here and looking for help!

Right I've done many a corporate identity over the years but I have a new problem, one which I haven't encountered before.....

I can't mention names as the work hasn't been released yet. A bank asked me to produce a piece of type to compliment an existing identity. I choose several fonts and presented to see what direction they wanted to go. Then then choose one (Aksidenz Grot. Ext.) I then modified this font changing several elements so that it worked with the accompanying arabic which was supplied by client.... also so it was a bespoke piece fit for purpose... so, they love it! Excellent job done.... Then they ask for a full working typeface based on my modified aksidenz....

What to do? As Ive modified heavily does this mean it's a new font, or do I have to go to Berthold and ask permission??? Not sure how this works, I want to do the job, but at the same time I don't want to break any law!!!!

ANY help would be really appreciated.

Thanks guys and girls!

M