When you dispose of an asset (be it by way of sale or through obsolescence) you have to calculate a balancing allowance / charge.
So in your case the fridge has a "tax written down value" of €350, and on disposal of it you received (presumably) Zero for it - your balancing allowance is €350 (this is an additional wear & tear allowance in the year of disposal).
The new fridge has nothing to do with the old one, you have acquired a new asset and it attracts wear & tear in the normal way.