Very bad service at Build it,

I engaged in the construction of a house and sourced materials from Build It. Typically, I would transfer funds to the store's bank account for them to deliver the materials to my Bulwer construction site, with our communication limited to emails and phone calls as I was in Durban. Upon completion, I requested a statement and found a balance of R1900, which was the store's owed refund to me. Despite this, I opted not to claim the refund immediately, planning to use it for future material purchases. However, when I revisited the store, they informed me that I could only utilize R1400, and the whereabouts of the remaining R500 were unknown. Having spent over R100,000 at the store, I am dismayed and frustrated by their customer service. The fact that they seem unwilling to address the issue and are seemingly withholding my R500 without a clear explanation is disappointing, and Im so ****ed.