First, be sure you have the correct foot for your specific machine.There can be different models of the foot for different machines.
What do you mean about the 7mm foot doesn't fit? Does it connect onto the shank holder on the machine? Does it need an adapter screw to hold it?
Your dilemma is the same reason I now have 3 of the 1/4 inch feet. I thought I lost one and bought a replacement. Then found the first one. Then bought a 3rd one--- a spare to the spare. I hope you find the original one soon. Good luck!