There seems to be more and more of this "Contract" work for people in the helth care field, yet the rules for determining Employed versus self-employed is one of the trickiest and costly areas of tax law. I would suggest that you consult a tax profesional to determine if your situation qualifies as self employed.
That being said, read the excise tax act to determine your OBLIGATIONS with respect to HST. I expect that if you charged your client over $30k in one year then you are required to: 1) Register for HST (obtain an HST number and account with CRA). 2) charge HST on your invoices, and display your HST registration number for all invoices over $25, and 3) file periodic HST returns (frequency determined by the amount paid) and pay the amounts owing.
You may also claim ITC's to reduce the amount owing.
I am not sure what you mean by collectibles, you may collect HST from your client if you invoice them for the amount, but this all ends up getting remitted to CRA (less input tax credits).
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