Salary Claims

Hi I am seeking an advice, I recently resign from my 3 yrs work previous employer. And now working here abroad. But before I left the country I was expecting to receive a back pay/salary claims worth Php50k but on the day they release my last pay I only receive is Php10k but in the documents that payroll showed me says I will be receiving Php50k and even ask me to sign the receiving documents with that certain amount. And now I am claiming the rest of my last pay to my previous employer and refusing to pay it as they were claiming they made a mistake on putting the amount that it should only be 10k but on the last time we talk they didn't mention it. And now I am filing a legal complaint on labor, on the first day sENA the previous employment came with a lawyer saying that I submit a voluntary resignation and will not be eligible for separation pay. I am not complaining for a separation pay what I am after is for my back pay on which indicates on the document they presented and have me signed it. So they refused it the sENA terminate the mediation process and recommended me to file a complaint against to my previous employer. Can anyone help me how will I defend myself as I can't because the previous party has a lawyer and I don't have. Can I still file for a complaint to get the rest of my last pay with my previous company. Your assistance will be much appreciated.