Matlab: generate matrix of random values with uniform distribution

i need to generate an array of dimension 20 per 20 of values uniformly distribuited in the interval [0,120] and mean=15, and which sum of all its values don't exceed 7000. i know how to generate this array respecting the first two conditions, but how about putting a constraint on the sum of its values?

Anyone can help, please? Thank you in advance.