From the FAQ. Also numbers 1.3.18, 1.4.18, and 1.5.17

In Account Siphon's "if successful" ability, the gaining of credits and the taking of tags are part of the same effect and thus resolve at the same time. The Runner cannot use those gained credits to avoid those taken tags with a New Angeles City Hall.

Page 10, Column 1, Paragraph(s) 5, FAQ
Can the Runner use New Angeles City Hall to avoid the tags from an Account Siphon if they have 0 credits when the ability resolves?
No. The gaining of the credits and the taking of the tags resolves at the same time, so the Runner cannot use those credits to avoid the tags.
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