From Google's dictionary:
The process of growth or increase, typically by the gradual accumulation of additional layers or matter
- the accretion of sediments in coastal mangroves
- the growing accretion of central government authority
A thing formed or added by such growth or increase
- about one-third of California was built up by accretions
- the city has a historic core surrounded by recent accretions
The coming together and cohesion of matter under the influence of gravitation to form larger bodies
So the sentence in question is wrong for two reasons; firstly, accretion is a noun, whereas your sentence uses it as an adjective. Secondly, accretion seems to refer to natural buildup, not the act of intentionally saving (from your example) money.
I think the word that you're looking for might be
accrue. From Merriam-Webster, a definition of
To accumulate or be added periodically (interest accrues on a daily basis)
So, you could say:
James bought this car using the accrued amount he had saved.